This invention relates to surface acoustic wave (SAW) signal processing elements having parabolically tapered horns to reduce the acoustic beamwidth of a surface acoustic wave and more particularly to such horns which include means to maintain a coherent wave front at the outlet of the horn.
Surface acoustic wave signal processing elements known as convolvers are becoming an important component in the design of modern communications systems. One type that directly utilizes the acoustic nonlinearities of a piezoelectric substrate, and is known as a SAW elastic convolver, shows great promise for use in high frequency, wide bandwidth difficult environment systems. An example of this type of convolver was reported by R. A. Becher and D. H. Hurlburt at pages 729-731 of the Proceeding of the 1979 Ultrasonics Symposium. This convolver, which will be more particularly described with respect to FIG. 1 in the Description of the Preferred Embodiment below, generally comprises a set of opposing parabolically tapered horns to reduce the acoustic beamwidth of waves incident thereon and which are coupled thereto from acoustic wave generating transducers, the output from the horns being coupled to a narrow interaction channel where the actual signal convolution is accomplished. The beam compression increases the acoustic power density in the interaction channel so as to increase the convolution efficiency. The circuit elements described above are generally in the form of microstrip on a piezoelectric substrait, typically lithium niobate.
An essential characteristic of the horn design is that the propagation time of all acoustic waves therethrough be identical within extremely close limits, otherwise phase incoherence of the wave exiting the horn structure will result. Phase incoherence produces distortion of the transmission bandwidth characteristic and an effective reduction in convolver efficiency, neither of which is desirable.
Generally, the acoustic wave coupled to the input of a horn structure has a straight or coherent wave front which is perpendicular to longitudinal axis of the horn structure. The wave front coherence is essentially maintained as the wave traverses the horn with structure without interruption. Thus, the central rays of the wave will move in a straight line from the horn input end to its output end. However, rays to either side of the central rays will intercept the sides of the parabolic horn and will be reflected therefrom to the horn output end. These reflected rays will travel a longer distance through the horn structure from input to output than the central rays which reach the horn output without reflection. Thus, the central rays will reach the horn output end before the reflected rays. This difference in transit times between the central rays and the other portions of the wave is one cause of phase dispersion which undesirably reduces bandwidth characteristic and convolver efficiency. The amount of phase dispersion depends on the percentage of wave energy in the central rays, which in turn is dependent on the horn beam compression. In the typical convolver horn the input end is ten times wider than the output end, hence the central ray will contain 10% of the total wave energy. The phase shift of the wave intercepted ray with respect to the central ray at the horn output end is dependent on the horn transit time of the wall intercept ray with respect to the horn transit time of the central ray. For a parabolic horn the phase difference is also equivalent to twice the ray transit time between the focus and vertex of the horn. For practical devices operating at normal frequencies this will typically be about 90.degree..